1. Field of the Invention
The present invention relates to a variable optical element that is capable of, for example, altering optical characteristics such as diffraction efficiency, a pickup apparatus that comprises the variable optical element, and an information recording and reproducing apparatus that comprises the pickup apparatus to carry out the recording of information to and reproduction from information recording media.
2. Description of the Related Art
In recent years, information recording media having recordable properties other than information recording media having reproduction-only properties and of a phase-change type have received attention as high-density and large-capacity information recording media. Among the phase-change type information recording media, write-once optical discs that allow one-time recording of information and rewritable optical discs that allow erasing and rewriting of information have been received attention.
These phase-change type information recording media have a layered structure, as shown in the sectional view of, for example, comprising a light-incident layer onto which a light beam is made incident from an objective lens provided in a so-called pickup apparatus, a recording layer of a crystal or amorphous material protected by a protection layer, a reflective layer, and a substrate, and are so configured that information is recorded by effecting phase changes to the recording layer by the light energy of a light beam. Reproduction of information is carried out by first irradiating a light beam onto the recording layer, which reflects the beam, then converging rays of reflected light with the objective lens, and finally detecting the reflected light with a photodetector of the pickup apparatus.
Write-once optical discs have a characteristic whereby they are capable of recording information only once, which is provided for by having a recording layer whose phase is changed (an irreversible change) according to light energy. Erasable optical discs have a characteristic whereby they are capable of rewriting information, which is provided for by having a recording layer whose phase is changed (a reversible change) according to light energy. In addition, the recording density of these information recording media are increased by narrowing the track pitch of the recording layer and further increases in recording capacity are rendered by adding more recording layers.
It is considered, on the other hand, that a light beam of a smaller focused spot diameter is irradiated onto a recording layer by using a greater numerical aperture (NA) of an objective lens provided in a pickup apparatus in order to realize an information recording/rewriting apparatus that keeps pace with the above mentioned high-density information recording media.
However, while increasing the numerical aperture (NA) of the objective lens corresponding to narrower track pitches of information recording media, it is important to carry out high-density information recording and appropriate information reproduction within the limited range of the numerical aperture (NA) of the objective lens. As a response to this problem, attention has been given to the usefulness of applying a crosstalk cancellation technique when reproducing information.
A crosstalk cancellation technique is such that not only a light beam is (referred to as the main beam) irradiated onto a track in which information is recorded (referred to as the main track) to read the information from it but also light beams (referred to as the sub-beams) are irradiated onto the tracks next to the main track (referred to as the adjacent tracks) to read information from them as well, whereupon by executing a predetermined calculation based upon the main signal obtained from the main beam and the sub-signals obtained from the sub-beams, a crosstalk component contained in the main signal is controlled to reproduce information with a good accuracy.
In addition, a method is considered to diffract a laser beam emitted from a laser light source with the use of a diffraction grating in order to generate the above mentioned main beam and sub-beams, and thereby irradiate the 0th order light as the main beam and +/xe2x88x92 1st order light as the sub-beams onto each corresponding track through an objective lens.
However, while above mentioned crosstalk canceling technique is applied to the reproduction of information from a reproduction-only information recording medium such as a DVD-ROM (digital versatile disc read only memory), which proves effective in reproducing information with good accuracy, if it is applied to the reproduction of information recorded on a phase-change type information recording medium such as a DVD-RW (digital versatile disc rewritable), it poses problems as described below.
In the case of a phase-change type information recording medium, the phase of the recording layer alters corresponding to the amount of light energy exerted. For this reason a problem is caused where, when reproducing information from a phase-change type information recording medium, irradiating the above mentioned main beam and sub-beams used for a reproduction-only information recording medium against the corresponding main track and sub-tracks of the phase-change type information recording medium imparts phase changes to the recording layer of the main track and/or sub-tracks to erase or destroy information that was recorded. In other words, a problem occurs where, if the same light beam is applied to both a reproduction-only information recording medium and a phase-change type information recording medium, it is suitable for one type of information recording medium but not suitable for the other type of information recording medium.
To avoid such a problem, a method can be considered to adjust the light energy levels of the main beam and sub-beams, by replacing the diffraction grating with another having a different diffraction efficiency for each particular case, depending on whether information is recorded to a phase-change type information recording medium, or information is reproduced from a phase-change type information recording medium, or where information is reproduced from a reproduction-only information recording medium which is not of a phase-change type. However, this method will give way to a problem in which a rather large-scale mechanism is required to replace one diffraction grating with another.
In addition, replacing the grating causes another problem where optical characteristics of the optical system including the above mentioned objective lens may become unstable.
Further, since replacement of the diffraction grating will take some time, when, for instance, recording and reproducing information to and from a phase-change type information recording medium is repeated one after the other, it will give rise to the problem of responsiveness in that recording and reproduction of information may not be carried out rapidly.
The present invention is made to overcome such conventional problems and holds an objective to provide a variable optical element that allows appropriate recording and reproducing of information to and from an information recording medium, an apparatus having such a variable optical element, and an information recording/reproducing apparatus having such a pickup apparatus.
Additionally, if a crosstalk cancellation technique is applied, it is another objective of the present invention to provide, a variable optical element that provides for performing information recording and reproducing with compatible with both a phase-change type information recording medium and a reproduction-only information recording medium, a pickup apparatus, and an information recording/reproducing apparatus.
In a first aspect of the invention, a variable optical element takes the structure wherein the first area having a piezo-electric medium layer with a piezo-electric effect and the second area that does not have the piezo-electric layer are formed on the top surface of a reference medium and imparts optical changes to the wave front of the light incident on the first and the second areas and reflects it based upon changes in the optical characteristics of the first and the second areas which are caused by the piezo-electric effect of the piezo-electric medium layer.
In a second aspect of the invention, a variable optical element comprises a piezo-electric medium layer with a piezo-electric effect, which has at least a first area and a second area with different thicknesses, and which imparts optical changes to the wave front of the light incident on at least the first and the second areas and reflects it based upon changes in optical characteristics which are caused by the piezo-electric effect of the piezo-electric medium layer of at least the first and the second areas.
According to the first or the second aspect of the invention, when a piezo-electric medium layer is distorted by the piezo-electric effect, optical characteristics of the first and the second areas are changed, a variable optical element causes optical changes to the wave front of the light incident on the first and the second areas and reflects it. Thus, by controlling the piezo-electric effect of a piezo-electric layer, various changes can be imparted to the incident light, which, for example, allows such effects as the appropriate control of the optical characteristics of a light beam irradiated onto an information recording medium for the purpose of recording or reproducing information.
In a third aspect of the invention, the variable optical element according to the first or the second aspect further has a plurality of pairs of the first and the second areas formed in a cyclic manner.
The variable optical element with such a structure also causes optical changes to the wave front of the light incident on the first and the second areas and reflects it when a piezo-electric medium layer is distorted by the piezo-electric effect, the optical characteristics of the first and the second areas are changed. Based on this behavior, by controlling the piezo-electric effect of a piezo-electric layer, various changes can be imparted to the incident light, which, for example, allows such uses as to appropriately control the optical characteristics of light beam irradiated onto an information recording medium for the purpose of recording or reproducing information.
In a fourth aspect of the invention, the piezo-electric medium layer of the variable optical element according to the first or the third aspect above changes in thickness as a result of the piezo-electric effect corresponding to voltages externally applied and alters the diffraction efficiency for the light incident on the first and the second areas based upon phase changes in the first and second areas which are imparted by the changes in thickness.
The variable optical element with such a structure also causes optical changes to the wave front of the light incident on the first and the second areas and reflects it when a piezo-electric medium layer is distorted by the piezo-electric effect, the optical characteristics of the first and the second areas are changed. Based on this behavior, by controlling the piezo-electric effect of a piezo-electric layer with electricity, various changes can be made to the incident light, which, for example, allows such uses as to the appropriate control of the optical characteristics of a light beam irradiated onto an information recording medium for the purpose of recording or reproducing information.
In a fifth aspect of the invention, a pickup apparatus which comprises the variable optical element according to the fourth aspect, irradiates a light beam for recording information onto an information recording medium or a light beam for reproducing information from an information recording medium onto the information recording medium, wherein a light source that emits light toward the first and the second areas of the variable optical element, and an optical system to generate the light beam for recording information or the light beam for reproducing information based upon diffracted and non-diffracted light rays that are caused when the variable optical element diffracted the light emitted against it are applied.
In the pickup apparatus of the invention having such a structure, based upon diffracted and non-diffracted light rays that are effected when a variable optical element diffracts the light emitted from a light source, an optical system generates a light beam for recording information or a light beam for reproducing information to irradiate it onto an information recording medium. Based on this, by controlling an electric field or a voltage to be applied on a variable optical element, for example, it is possible to generate a light beam having power suitable for recording information on an information recording medium or a light beam having power suitable for reproducing information from an information recording medium. In a sixth aspect of the invention, the pickup apparatus according to the fifth aspect further has a photodetector to detect reflected light which is effected when the information recording medium reflects the light beam.
In the pickup apparatus with such a structure, the photodetector detects reflected light, which is originally a light beam for recording information irradiated to an information recording medium and thereby reflected by it, and the photodetector provides for the generation of control signals for processing an appropriate recording of information based on the results of detection. In addition, this photodetector detects reflected light, which is originally a light beam for reproducing information irradiated to an information recording medium and thereby reflected by it, and the photodetector provides for the generation of control signals for processing appropriate information reproduction based on the results of detection.
In a seventh aspect of the invention, an information recording/reproducing apparatus contains the pickup apparatus according to the sixth aspect above and, has a means to control at least the voltages applied to the piezo-electric medium layer between the power of the light emitted from the light source and the voltages applied to the piezo-electric medium layer.
In an eighth aspect of the invention, the control means of the information recording/reproducing apparatus according to the seventh aspect above, at least when recording information to the information recording medium with the light beam for recording information, controls the diffraction efficiency so that the light beam power of the diffracted light assumes a level that does not erase information on the information recording medium by setting the voltage applied to the piezo-electric medium layer to a predetermined voltage level.
According to the information recording/reproducing apparatus in the seventh or eighth aspect, it is possible to variably control the diffraction efficiency of a variable optical element by adjusting the voltages applied to a piezo-electric medium layer. Based on this, it is possible to eliminate problems where information on an information recording medium is unnecessarily erased by appropriately adjusting the power of the light beam for recording information to or reproducing information from an information recording medium.
In a ninth aspect of the invention, the information recording/reproducing apparatus according to the eighth aspect further contains a crosstalk canceller circuit that suppresses crosstalk components based on the information output by the photodetector while it detects the reflected light.
An information recording/reproducing apparatus having such a construction provides for the reproduction of information with suppressed crosstalk components based on the detected results detected by the photodetector while reproducing information.